This invention relates to fuel cell systems.
A fuel cell can convert chemical energy to electrical energy by promoting a chemical reaction between two gases.
One type of fuel cell includes a cathode flow field plate, an anode flow field plate, a membrane electrode assembly disposed between the cathode flow field plate and the anode flow field plate, and two gas diffusion layers disposed between the cathode flow field plate and the anode flow field plate. A fuel cell can also include one or more coolant flow field plates disposed adjacent the exterior of the anode flow field plate and/or the exterior of the cathode flow field plate.
Each flow field plate has an inlet region, an outlet region and open-faced channels connecting the inlet region to the outlet region and providing a way for distributing the gases to the membrane electrode assembly.
The membrane electrode assembly usually includes a solid electrolyte (e.g., a proton exchange membrane, commonly abbreviated as a PEM) between a first catalyst and a second catalyst. One gas diffusion layer is between the first catalyst and the anode flow field plate, and the other gas diffusion layer is between the second catalyst and the cathode flow field plate.
During operation of the fuel cell, one of the gases (the anode gas) enters the anode flow field plate at the inlet region of the anode flow field plate and flows through the channels of the anode flow field plate toward the outlet region of the anode flow field plate. The other gas (the cathode gas) enters the cathode flow field plate at the inlet region of the cathode flow field plate and flows through the channels of the cathode flow field plate toward the cathode flow field plate outlet region.
As the anode gas flows through the channels of the anode flow field plate, the anode gas passes through the anode gas diffusion layer and interacts with the anode catalyst. Similarly, as the cathode gas flows through the channels of the cathode flow field plate, the cathode gas passes through the cathode gas diffusion layer and interacts with the cathode catalyst.
The anode catalyst interacts with the anode gas to catalyze the conversion of the anode gas to reaction intermediates. The reaction intermediates include ions and electrons. The cathode catalyst interacts with the cathode gas and the reaction intermediates to catalyze the conversion of the cathode gas to the chemical product of the fuel cell reaction.
The chemical product of the fuel cell reaction flows through a gas diffusion layer to the channels of a flow field plate (e.g., the cathode flow field plate). The chemical product then flows along the channels of the flow field plate toward the outlet region of the flow field plate.
The electrolyte provides a barrier to the flow of the electrons and gases from one side of the membrane electrode assembly to the other side of the membrane electrode assembly. However, the electrolyte allows ionic reaction intermediates to flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly.
Therefore, the ionic reaction intermediates can flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly without exiting the fuel cell. In contrast, the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly by electrically connecting an external load between the anode flow field plate and the cathode flow field plate. The external load allows the electrons to flow from the anode side of the membrane electrode assembly, through the anode flow field plate, through the load and to the cathode flow field plate.
Electrons are formed at the anode side of the membrane electrode assembly, indicating that the anode gas undergoes oxidation during the fuel cell reaction. Electrons are consumed at the cathode side of the membrane electrode assembly, indicating that the cathode gas undergoes reduction during the fuel cell reaction.
For example, when hydrogen and oxygen are the gases used in a fuel cell, the hydrogen flows through the anode flow field plate and undergoes oxidation. The oxygen flows through the cathode flow field plate and undergoes reduction. The specific reactions that occur in the fuel cell are represented in equations 1–3.H2→2H++2e−  (1)½O2+2H++2e−→H2O  (2)H2+½O2→H2O  (3)
As shown in equation 1, the hydrogen is reacted to form protons (H+) and electrons. The protons flow through the electrolyte to the cathode side of the membrane electrode assembly, and the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly through the external load. As shown in equation 2, the electrons and protons react with the oxygen to form water. Equation 3 shows the overall fuel cell reaction.
In addition to forming chemical products, the fuel cell reaction produces heat. One or more coolant flow field plates are typically used to conduct the heat away from the fuel cell and prevent it from overheating.
Each coolant flow field plate has an inlet region, an outlet region and channels that provide fluid communication between the coolant flow field plate inlet region and the coolant flow field plate outlet region. A coolant (e.g., liquid de-ionized water, or other non conducting fluid) at a relatively low temperature enters the coolant flow field plate at the inlet region, flows through the channels of the coolant flow field plate toward the outlet region of the coolant flow field plate, and exits the coolant flow field plate at the outlet region of the coolant flow field plate. As the coolant flows through the channels of the coolant flow field plate, the coolant absorbs heat formed in the fuel cell. When the coolant exits the coolant flow field plate, the heat absorbed by the coolant is removed from the fuel cell.
FIG. 1 shows a fuel cell system 300 including a fuel cell assembly 302 having a plurality of fuel cells 304. Fuel cell system 300 also includes an anode gas supply 306, an anode gas inlet line 308, an anode gas outlet line 310, a cathode gas supply 312, a cathode gas inlet line 314, a cathode gas outlet line 316, a coolant inlet line 318, and a coolant outlet line 320.
To increase the electrical energy available, the plurality of fuel cells 304 can be arranged in series, to form a fuel cell stack 303. For example, one side of a flow field plate functions as the anode flow field plate for one fuel cell while the opposite side of the flow field plate functions as the cathode flow field plate in another fuel cell. This arrangement may be referred to as a bipolar plate. The stack may also include plates such as, for example, an anode coolant flow field plate having one side that serves as an anode flow field plate and another side that serves as a coolant flow field plate, e.g., a monopolar plate. As an example, the open-faced coolant channels of an anode coolant flow field plate and a cathode coolant flow field plate may be mated to form collective coolant channels to cool the adjacent flow field plates forming fuel cells.
Fuel cell stack 303 is typically provided with inlets, outlets, and manifolds for directing the flow of reactants and coolant to the appropriate flow plates, and assembled between a pair of thick rigid end plates, which are also provided with inlets and outlets. The end plate that is used to deliver one or more reactants to an end of the fuel cell stack is sometimes called a service end plate 321 (FIG. 2). The end plate at the opposite end of the fuel cell stack is sometimes called a blind end plate 323 (FIG. 2). The edges of the two end plates are bolted together to apply a compressive force on the fuel cell stack.
FIG. 2 is a partial schematic representation of fuel cell system 300 in operation. Anode gas supply 306, e.g., a reformer, provides in parallel hydrogen gas via inlet line 308 to the anodes of cells 1 through N, e.g., 88. At each cell, the anode converts the hydrogen into protons and electrons. The protons travel through the solid electrolyte and to the cathode of the respective cells. At cell 1, the electrons flow toward an external load. At the other cells, the electrons flow to the cathode of an adjacent fuel cell, toward the external load. Unreacted anode gas flows through the cells of fuel cell stack 303 through outlet 310.
Similarly, cathode gas supply 312, e.g., an air blower, provides in parallel oxygen (air) via inlet line 314 to the cathode of cells 1 through N. At each cell, the cathode forms water from the oxygen, protons from the respective anode, and electrons flowing from the external load (cell N) or adjacent anode (cell 1 through N−1). The water can be removed from stack 303 by the cathode gas stream. After flowing through the cells, the oxygen flows out of fuel cell stack 303 through outlet 316.
Thus, as the anode and cathode gases are supplied to fuel cell system 300, hydrogen and oxygen are converted into water, and electrons flow through the external load, thereby supplying electrical energy.